Prolonged space flight-induced alterations in the structure and function of human skeletal muscle fibres.

The primary goal of this study was to determine the effects of prolonged space flight (180 days) on the structure and function of slow and fast fibres in human skeletal muscle. Biopsies were obtained from the gastrocnemius and soleus muscles of nine International Space Station crew members 45 days pre- and on landing day (R+0) post-flight. The main findings were that prolonged weightlessness produced substantial loss of fibre mass, force and power with the hierarchy of the effects being soleus type I > soleus type II > gastrocnemius type I > gastrocnemius type II. Structurally, the quantitatively most important adaptation was fibre atrophy, which averaged 20% in the soleus type I fibres (98 to 79 µm diameter). Atrophy was the main contributor to the loss of peak force (P(0)), which for the soleus type I fibre declined 35% from 0.86 to 0.56 mN. The percentage decrease in fibre diameter was correlated with the initial pre-flight fibre size (r = 0.87), inversely with the amount of treadmill running (r = 0.68), and was associated with an increase in thin filament density (r = 0.92). The latter correlated with reduced maximal velocity (V(0)) (r = 0.51), and is likely to have contributed to the 21 and 18% decline in V(0) in the soleus and gastrocnemius type I fibres. Peak power was depressed in all fibre types with the greatest loss (55%) in the soleus. An obvious conclusion is that the exercise countermeasures employed were incapable of providing the high intensity needed to adequately protect fibre and muscle mass, and that the crew's ability to perform strenuous exercise might be seriously compromised. Our results highlight the need to study new exercise programmes on the ISS that employ high resistance and contractions over a wide range of motion to mimic the range occurring in Earth's 1 g environment.

The voltage sensor of excitation-contraction coupling in skeletal muscle. Ion dependence and selectivity.

Manifestations of excitation-contraction (EC) coupling of skeletal muscle were studied in the presence of metal ions of the alkaline and alkaline-earth groups in the extracellular medium. Single cut fibers of frog skeletal muscle were voltage clamped in a double Vaseline gap apparatus, and intramembrane charge movement and myoplasmic Ca2+ transients were simultaneously measured. In metal-free extracellular media both charge movement of the charge 1 type and Ca transients were suppressed. Under metal-free conditions the nonlinear charge distribution was the same in depolarized (holding potential of 0 mV) and normally polarized fibers (holding potentials between -80 and -90 mV). The manifestations of EC coupling recovered when ions of groups Ia and IIa of the periodic table were included in the extracellular solution; the extent of recovery depended on the ion species. These results are consistent with the idea that the voltage sensor of EC coupling has a binding site for metal cations--the "priming" site--that is essential for function. A state model of the voltage sensor in which metal ligands bind preferentially to the priming site when the sensor is in noninactivated states accounts for the results. This theory was used to derive the relative affinities of the various ions for the priming site from the magnitude of the EC coupling response. The selectivity sequence thus constructed is: Ca greater than Sr greater than Mg greater than Ba for group IIa cations and Li greater than Na greater than K greater than Rb greater than Cs for group Ia. Ca2+, the most effective of all ions tested, was 1,500-fold more effective than Na+. This selectivity sequence is qualitatively and quantitatively similar to that of the intrapore binding sites of the L-type cardiac Ca channel. This provides further evidence of molecular similarity between the voltage sensor and Ca channels.

The effect of ageing and exercise on skeletal muscle function.

The contractile and selected biochemical properties of fast- and slow-twitch skeletal muscle were studied at 9, 18, and 28 months of age in sedentary and regularly exercised rats. The isometric twitch duration was prolonged with aging in both the fast- and slow-twitch muscle. This effect was primarily due to a prolonged one-half relaxation time (1/2RT), which developed late in life. Regular exercise tended to further prolong the twitch duration, particularly in the slow-twitch soleus. Surprisingly, twitch and tetanic tension (Po), peak rate of tension development and decline, and the maximal shortening velocity were all unaltered between 9 and 28 months of age. Furthermore, regular exercise (running or swimming) had little or no effect on these properties. The prolonged 1/2RT with aging could not be explained by a decreased rate of Ca2+ sequestration by the sarcoplasmic reticulum, as the rate of Ca2+ uptake measured in muscle homogenates was unaltered in any of the muscles studied between 9 and 28 months. The degree of muscle fatigue (decline in Po) with 30 min of contractile activity in the slow-twitch soleus was not affected by aging. However, lactate reached two-fold higher levels and glycogen fell to considerably lower levels in the muscles of the old rats. This suggests an increased glycolysis and glycogen utilization during contractile activity in aged rats.

Role of intracellular pH in muscle fatigue.

Intracellular pH of in vitro diaphragm preparations was determined following low- (5 Hz, 1.5 min) and high- (75 Hz, 1 min) frequency stimulation, using glass microelectrodes of the liquid membrane type (pHm). Results were compared with values obtained by the standard homogenate technique (pHh). High- and low-frequency stimulation reduced peak tetanic tension to 21 +/- 1 (SE) and 71 +/- 2% of initial values, respectively. Peak tetanic tension returned to resting values after 10- to 15-min recovery from high- or low-frequency stimulation. Resting pHm was 7.063 +/- 0.011 (n = 72), and after fatiguing stimulation declined to values as low as 6.33. During recovery pHm significantly increased and by 10 min had returned to prefatigue values. No difference was observed in the recovery of pHm between the low- and high-frequency stimulation groups (analysis of variance test, ANOVA), and in both groups pHm recovery was highly correlated to the recovery of peak tetanic tension (r = 0.94, P less than 0.001). Resting pHh was 7.219 +/- 0.023 (n = 13), which was significantly higher than the pHm value. In contrast to pHm, intracellular pHh was significantly higher during recovery from 75- vs. 5-Hz stimulation (P less than 0.05). For both groups pHh increased significantly with time and by 10 min returned to prestimulation values. The ANOVA test demonstrated that pHh values were significantly higher than pHm values during recovery from fatigue. The results from this study support our hypothesis that fatigue from both high- and low-frequency stimulation is at least partially due to the deleterious effects of intracellular acidosis on excitation-contraction coupling.

Fatigue from high- and low-frequency muscle stimulation: contractile and biochemical alterations.

This study examined the effect of high- (75 Hz, 1 min) and low- (5 Hz, 1.5 min) frequency stimulation on contractile and biochemical properties of the diaphragm. Tension was reduced to 21 +/- 1 and 54 +/- 2% (SE) of the initial value after high- and low-frequency stimulation, respectively. After 0, 0.25, 1, and 2 min of recovery from high-frequency stimulation, 5 Hz elicited more force (expressed as % of initial tension) than 75-Hz stimulation. Time 0 recovery values were 21 +/- 1 and 78 +/- 6% of the initial force for 75- and 5-Hz stimulation, respectively. By 1 min of recovery, force elicited by 5-Hz stimulation had returned to the prefatigue value. In contrast, force production with 75-Hz stimulation did not full recover until 10-15 min. After fatigue produced by low-frequency stimulation, force production with 5-Hz stimulation was reduced to 54 +/- 2% of the initial tension, a value significantly lower than the 71 +/- 2% of initial force elicited by 75-Hz stimulation. Force production with 5-Hz stimulation increased rapidly in the first 15 s of recovery (54 +/- 2% at 0 and 70 +/- 2% at 15 s) and by 5 min was significantly greater than the force elicited by 75-Hz stimulation (100 +/- 3 vs. 93 +/- 1%). As before, force production at 75-Hz stimulation did not fully recover until 10-15 min. Both fatigue protocols produced a significant prolongation in isometric twitch contraction and one-half relaxation times. Creatine phosphate (CP) concentration was reduced and muscle lactate increased by both fatigue protocols.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of depolarization and low intracellular pH on charge movement currents of frog skeletal muscle fibers.

The low intracellular pH and membrane depolarization associated with repeated skeletal muscle stimulation could impair the function of the transverse tubular (t tubule) voltage sensor and result in a decreased sarcoplasmic reticulum Ca(2+) release and muscle fatigue. We therefore examined the effects of membrane depolarization and low intracellular pH on the t-tubular charge movement. Fibers were voltage clamped in a double Vaseline gap, at holding potential (HP) of -90 or -60 mV, and studied at an internal pH of 7.0 and 6.2. Decreasing intracellular pH did not significantly alter the maximum amount of charge moved, transition voltage, or steepness factor at either HP. Depolarizing HP significantly decreased steepness factor and maximum charge moved and shifted the transition voltage to more positive potentials. Elevated extracellular Ca(2+) decreased the depolarization-induced reduction in the charge movement. These results indicate that, although the decrease in intracellular pH seen in fatigued muscle does not impair the t-tubular charge movement, the membrane depolarization associated with muscle fatigue may be sufficient to inactivate a significant fraction of the t-tubular charge. However, if t-tubular Ca(2+) increases, some of the charge may be stabilized in the active state and remain available to initiate sarcoplasmic reticulum Ca(2+) release.

Hindlimb unloading-induced muscle atrophy and loss of function: protective effect of isometric exercise.

The primary objective of this study was to determine the effectiveness of isometric exercise (IE) as a countermeasure to hindlimb unloading (HU)-induced atrophy of the slow (soleus) and fast (plantaris and gastrocnemius) muscles. Rats were assigned to either weight-bearing control, 7-day HU (H7), H7 plus IE (I7), 14-day HU (H14), or H14 plus IE (I14) groups. IE consisted of ten 5-s maximal isometric contractions separated by 90 s, administered three times daily. Contractile properties of the soleus and plantaris muscles were measured in situ. The IE attenuated the HU-induced decline in the mass and fiber diameter of the slow-twitch soleus muscle, whereas the gastrocnemius and plantaris mass were not protected. These results are consistent with the mean electromyograph recordings during IE that indicated preferential recruitment of the soleus over the gastrocnemius and plantaris muscles. Functionally, the IE significantly protected the soleus from the HU-induced decline in peak isometric force (I14, 1.49 +/- 0.12 vs. H14, 1.15 +/- 0.07 N) and peak power (I14, 163 +/- 17 vs. H14, 75 +/- 11 mN.fiber length.s-1). The exercise protocol showed protection of the plantaris peak isometric force at H7 but not H14. The IE also prevented the HU-induced decline in the soleus isometric contraction time, which allowed the muscle to produce greater tension at physiological motoneuron firing frequencies. In summary, IE resulted in greater protection from HU-induced atrophy in the slow soleus than in the fast gastrocnemius or plantaris.

Peak force and maximal shortening velocity of soleus fibers after non-weight-bearing and resistance exercise.

This study examined the effectiveness of resistance exercise as a countermeasure to non-weight-bearing-induced alterations in the absolute peak force, normalized peak force (force/fiber cross-sectional area), peak stiffness, and maximal shortening velocity (Vo) of single permeabilized type I soleus muscle fibers. Adult rats were subjected to the following treatments: normal weight bearing (WB), non-weight bearing (NWB), or NWB with exercise treatments (NWB+Ex). The hindlimbs of the NWB and NWB+Ex rats were suspended for 14 days via tail harnesses. Four times each day, the NWB+Ex rats were removed from suspension and performed 10 climbs (approximately 15 cm each) up a steep grid with a 500-g mass (approximately 1.5 times body mass) attached to their tail harness. NWB was associated with significant reductions in type I fiber diameter, absolute force, normalized force, and stiffness. Exercise treatments during NWB attenuated the decline in fiber diameter and absolute force by almost 60% while maintaining normalized force and stiffness at WB levels. Type I fiber Vo increased by 33% with NWB and remained at this elevated level despite the exercise treatments. We conclude that in comparison to intermittent weight bearing only (J.J. Widrick, J.J. Bangart, M. Karhanek, and R.H. Fitts. J. Appl. Physiol. 80: 981-987, 1996), resistance exercise was more effective in attenuating alterations in type I soleus fiber absolute force, normalized force, and stiffness but was less effective in restoring type I fiber Vo to WB levels.

Contractile and biochemical properties of diaphragm: effects of exercise training and fatigue.

The effect of high-intensity trained (6 X 4.5 min at 40 m/min, 15% grade, 2.5-min rest between bouts, 5 days/wk, for 6 wk) on contractile, biochemical, and fatigue properties of the rat diaphragm were examined. The exercise program produced significant elevations in the mitochondrial marker enzyme citrate synthase (mumol X g-1 X min-1) in the soleus (SOL) (27.2 +/- 1.5 vs. 46.7 +/- 2.4; mean +/- SE), deep vastus lateralis (DVL) (40.8 +/- 2.6 vs. 58.3 +/- 2.8), and superficial vastus lateralis (SVL) (8.5 +/- 0.6 vs. 11.4 +/- 0.7). No significant differences were observed in the crural (CRU) (45.9 +/- 2.0 vs. 44.0 +/- 2.3) or ventral costal (VEN) (41.5 +/- 2.0 vs. 45.8 +/- 2.6) diaphragmatic regions. Phosphofructokinase, the rate-limiting enzyme of glycolysis, significantly increased in the SOL (19.0 +/- 0.8 vs. 23.3 +/- 1.3 mumol X g-1 X min-1) and DVL (69.3 +/- 6.0 vs. 86.6 +/- 5.0), but no alterations were seen in the SVL (98.6 +/- 5.7 vs. 106.1 +/- 9.0), CRU (54.4 +/- 2.8 vs. 53.8 +/- 1.5), or VEN (44.7 +/- 2.4 vs. 46.4 +/- 1.4) posttraining. Diaphragm contractile properties, with the exception of an increased rate of fall in twitch tension, remained unchanged after training. Glycogen values were significantly higher in trained diaphragms at rest (6.54 +/- 0.39 vs. 4.86 +/- 0.41 mg/g) and during 1, 5, and 10 min of fatiguing stimulation. During fatigue no differences were observed in force, rate of rise in force, rate of fall in force, muscle lactate, ATP, or creatine phosphate in trained vs. control.(ABSTRACT TRUNCATED AT 250 WORDS)

Recovery in skeletal muscle contractile function after prolonged hindlimb immobilization.

Contractile properties of slow-twitch soleus (SOL), fast-twitch extensor digitorum longus (EDL), and fast-twitch superficial region of the vastus lateralis were determined in vitro (22 degrees C) in rats remobilized after prolonged (3 mo) hindlimb immobilization (IM). For all muscles the muscle-to-body weight ratio was significantly depressed by IM, and the ratios failed to completely recover even after 90 days. The contractile properties of the fast-twitch muscles were less affected by IM than the slow-twitch SOL. The IM shortened the SOL isometric twitch duration due to a reduced contraction and half-relaxation time. These parameters returned to control levels by the 14th day of recovery. Peak tetanic tension (Po, g/cm2) declined with IM by 46% in the SOL but showed no significant change in the fast-twitch muscles. After IM the SOL Po (g/cm2) recovered to control values by 28 days. The recovery of Po in absolute units (g) was considerably slower and did not return to control levels until 60 (SOL) to 90 (EDL) days. The maximum shortening velocity was not altered by IM in any of the muscles studied. These results demonstrate that both fast- and slow-twitch skeletal muscles possess the ability to completely recover normal contractile function following prolonged periods of hindlimb IM.

Contractile responses of the rat gastrocnemius and soleus muscles to isotonic resistance exercise.

Male rats were divided into control and weight-trained (WT) groups. WT rats performed squat-type exercises twice daily, 5 days/wk, for 14 wk. They averaged 36 lifts/day, with an average weight of 555 g. Muscle-to-body weight ratio (mg/g) of the soleus (Sol) was not different from control, but it increased 11 and 6% in the gastrocnemius (Gast) and plantaris, respectively (P < 0.05). The normalized twitch tension of the in situ Sol was elevated by 21%, whereas single-skinned type I fibers from the Sol showed an increased rate constant of tension redevelopment (K(tr)) but no other contractile adaptations to WT. In contrast, the Gast type I fibers showed an increase (P < 0.05) in maximal velocity of shortening (25%), peak power (15%), K(tr) (18%), and normalized tension (7%). The K(tr) and normalized tension of the Gast type IIa fibers increased by 24% (P < 0.05) and 12% (P < 0.05), respectively, whereas velocity and power showed a tendency to increase. Fiber size, determined by myosin ATPase histochemistry, was not different for any fiber type from the Gast or Sol. These results indicate that isotonic resistance exercise of the calf targets the Gast (type I and type IIa fibers) and has little effect on the Sol.

Effects of fatiguing stimulation on intracellular Na+ and K+ in frog skeletal muscle.

The purpose of this study was to describe the alterations in the intracellular concentrations of sodium ([Na+]i) and potassium ([K+]i) and the membrane potential (Em) as a result of fatiguing stimulation of the frog semitendinosus muscle and to relate these changes to the alterations in the sarcolemma action potential and force-generating ability of the muscle. [Na+]i and [K+]i were measured by using ion-selective microelectrodes. Before stimulation (100-ms trains at 150 Hz, 1 stimulus/s for 5 min), [Na+]i, [K+]i, and Em were 16 +/- 1 mM, 142 +/- 5 mM, and -83 +/- 1 mV, respectively. As a result of stimulation, [Na+]i rose to 49 +/- 6 mM and recovered to 16 +/- 2 mM with a time constant (tau) of 70 s.[K+]i fell to 97 +/- 8 mM as a result of stimulation, then recovered to 148 +/- 5 mM with tau = 56 s. Em depolarized to -74 +/- 3 mV then recovered to -83 +/- 2 mV with tau = 53 s. The Na+/K+ permeability ratio of the resting membrane fell 3%, whereas at the peak of the action potential the permeability ratio fell 38%. A previous study using the same muscle and stimulation protocol showed force to recover with a fast initial phase (approximately 2 min) and a much slower second phase (approximately 50 min). The recovery of [Na+]i, [K+]i, and Em was similar to the fast phase of force recovery; thus the altered Na+ and K+ concentration gradient across the sarcolemma and t-tubular membrane may contribute to this component of fatigue. The possible fatigue mechanisms induced by the altered ionic gradients include 1) complete block of the action potential propagation; 2) depolarization-induced inactivation of t-tubular charge movement; and 3) a reduced magnitude of the t-tubular charge due to the lower action potential spike potential.

Effect of hindlimb unweighting on single soleus fiber maximal shortening velocity and ATPase activity.

This study characterizes the time course of change in single soleus muscle fiber size and function elicited by hindlimb unweighting (HU) and analyzes the extent to which varying durations of HU altered maximal velocity of shortening (Vo), myofibrillar adenosinetriphosphatase (ATPase), and relative content of slow and fast myosin in individual soleus fibers. After 1, 2, or 3 wk of HU, soleus muscle bundles were prepared and stored in skinning solution at -20 degrees C. Single fibers were isolated and mounted between a motor arm and a transducer, and fiber force, Vo, and ATPase activity were measured. Fiber myosin content was determined by one-dimensional sodium dodecyl sulfate- (SDS) polyacrylamide gel electrophoresis. After 1, 2, and 3 wk of HU, soleus fibers exhibited a progressive reduction in fiber diameter (16, 22, and 42%, respectively) and peak force (42, 48, and 72%, respectively). Peak specific tension was significantly reduced after 1 wk of HU (18%) and showed no further change in 2-3 wk of HU. During 1 and 3 wk of HU, fiber Vo and ATPase showed a significant increase. By 3 wk, Vo had increased from 1.32 +/- 0.04 to 2.94 +/- 0.17 fiber lengths/s and fiber ATPase from 291 +/- 16 to 1,064 +/- 128 microM.min-1 x mm-3. The percent fibers expressing fast myosin heavy chain increased from 4% to 29% by 3 wk of HU, and Vo and ATPase activity within a fiber were highly correlated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hindlimb unloading on rat soleus fiber force, stiffness, and calcium sensitivity.

The purpose of this study was to examine the time course of change in soleus muscle fiber peak force (N), tension (Po, kN/m2), elastic modulus (Eo), and force-pCa and stiffness-pCa relationships. After 1, 2, or 3 wk of hindlimb unloading (HU), single fibers were isolated and placed between a motor arm and a transducer, and fiber diameter, peak absolute force, Po, Eo, and force-pCa and stiffness-pCa relationships were characterized. One week of HU resulted in a significant reduction in fiber diameter (68 +/- 2 vs 57 +/- 1 microns), force (3.59 +/- 0.15 vs. 2.19 +/- 0.12 x 10(-4) N), Po (102 +/- 4 vs. 85 +/- 2 kN/m2), and Eo (1.96 +/- 0.12 vs. 1.37 +/- 0.13 x 10(7) N/m2), and 2 wk of HU caused a further decline in fiber diameter (45 +/- 1 microns), force (1.31 +/- 0.06 x 10(-4) N), and Eo (0.96 +/- 0.09 x 10(7) N/m2). Although the mean fiber diameter and absolute force continued to decline through 3 wk of HU, Po recovered to values not significantly different from control. The Po/Eo ratio was significantly increased after 1 (5.5 +/- 0.3 to 7.1 +/- 0.6), 2, and 3 wk of HU, and the 2-wk (9.5 +/- 0.4) and 3-wk (9.4 +/- 0.8) values were significantly greater than the 1-wk values. The force-pCa and stiffness-pCa curves were shifted rightward after 1, 2, and 3 wk of HU. At 1 wk of HU, the Ca2+ sensitivity of isometric force, assessed by Ca2+ concentration required for half-maximal force, was increased from the control value of 1.83 +/- 0.12 to 2.30 +/- 0.10 microM. In conclusion, after HU, the decrease in soleus fiber Po can be explained by a reduction in the number of myofibrillar cross bridges per cross-sectional area. Our working hypothesis is that the loss of contractile protein reduces the number of cross bridges per cross-sectional area and increases the filament lattice spacing. The increased spacing reduces cross-bridge force and stiffness, but Po/Eo increases because of a quantitatively greater effect on stiffness.

Effect of intermittent weight bearing on soleus fiber force-velocity-power and force-pCa relationships.

Rat permeabilized type I soleus fibers displayed a 33% reduction in peak power output and a 36% increase in the free Ca2+ concentration required for one-half maximal activation after 14 days of hindlimb non-weight bearing (NWB). We examined the effectiveness of intermittent weight bearing (IWB; consisting of four 10-min periods of weight bearing/day) as a countermeasure to these functional changes. At peak power output, type I fibers from NWB animals produced 54% less force and shortened at a 56% greater velocity than did type I fibers from control weight-bearing animals while type I fibers from the IWB rats produced 26% more absolute force than did fibers from the NWB group and shortened at a velocity that was only 80% of the NWB group mean. As a result, no difference was observed in the average peak power of fibers from the IWB and NWB animals. Hill plot analysis of force-pCa relationships indicated that fibers from the IWB group required similar levels of free Ca2+ to reach half-maximal activation in comparison to fibers from the weight-bearing group. However, at forces < 50% of peak force, the force-pCa curve for fibers from the IWB animals clearly fell between the relationships observed for the other two groups. In summary, IWB treatments 1) attenuated the NWB-induced reduction in fiber Ca2+ sensitivity but 2) failed to prevent the decline in peak power that occurs during NWB because of opposing effects on fiber force (an increase vs. NWB) and shortening velocity (a decrease vs. NWB).

Effect of hindlimb unweighting on tissue blood flow in the rat.

The purpose of this study was to characterize the distribution of blood flow in the rat during hindlimb unweighting (HU) and post-HU standing and exercise and examine whether the previously reported (Witzmann et al., J. Appl. Physiol. 54: 1242-1248, 1983) elevation in anaerobic metabolism observed with contractile activity in the atrophied soleus muscle was caused by a reduced hindlimb blood flow. After either 15 days of HU or cage control, blood flow was measured with radioactive microspheres during unweighting, normal standing, and running on a treadmill (15 m/min). In another group of control and experimental animals, blood flow was measured during preexercise (PE) treadmill standing and treadmill running (15 m/min). Soleus muscle blood flow was not different between groups during unweighting, PE standing, and running at 15 m/min. Chronic unweighting resulted in the tendency for greater blood flow to muscles composed of predominantly fast-twitch glycolytic fibers. With exercise, blood flow to visceral organs was reduced compared with PE values in the control rats, whereas flow to visceral organs in 15-day HU animals was unaltered by exercise. These higher flows to the viscera and to muscles composed of predominantly fast-twitch glycolytic fibers suggest an apparent reduction in the ability of the sympathetic nervous system to distribute cardiac output after chronic HU. In conclusion, because 15 days of HU did not affect blood flow to the soleus during exercise, the increased dependence of the atrophied soleus on anerobic energy production during contractile activity cannot be explained by a reduced muscle blood flow.

Soleus fiber force and maximal shortening velocity after non-weight bearing with intermittent activity.

This study examined the effectiveness of intermittent weight bearing (IWB) as a countermeasure to non-weight-bearing (NWB)-induced alterations in soleus type I fiber force (in mN), tension (Po; force per fiber cross-sectional area in kN/m-2), and maximal unloaded shortening velocity (Vo, in fiber lengths/s). Adult rats were assigned to one of the following groups: normal weight bearing (WB), 14 days of hindlimb NWB (NWB group), and 14 days of hindlimb NWB with IWB treatments (IWB group). The IWB treatment consisted of four 10-min periods of standing WB each day. Single, chemically permeabilized soleus fiber segments were mounted between a force transducer and position motor and were studied at maximal Ca2+ activation, after which type I fiber myosin heavy-chain composition was confirmed by sodium dodecyl sufate-polyacrylamide gel electrophoresis. NWB resulted in a loss in relative soleus mass (-45%), with type I fibers displaying reductions in diameter (-28%) and peak isometric force (-55%) and an increase in Vo (+33%). In addition, NWB induced a 16% reduction in type I fiber Po, a 41% reduction in type I fiber peak elastic modulus [Eo, defined as (delta force/delta length) x (fiber length/fiber cross-sectional area] and a significant increase in the Po/Eo ratio. In contrast to NWB, IWB reduced the loss of relative soleus mass (by 22%) and attenuated alterations in type I fiber diameter (by 36%), peak force (by 29%), and Vo (by 48%) but had no significant effect on Po, Eo, or Po/Eo. These results indicate that a modest restoration of WB activity during 14 days of NWB is sufficient to attenuate type I fiber atrophy and to partially restore type I peak isometric force and Vo to WB levels. However, the NWB-induced reductions in Po and Eo, which we hypothesize to be due to a decline in the number and stiffness of cross bridges, respectively, are considerably less responsive to this countermeasure treatment.

Role of sarcolemma action potentials and excitability in muscle fatigue.

The purposes of this study were to characterize the alterations in the sarcolemma action potential (AP) waveform and sarcolemma excitability as a result of fatiguing stimulation of the frog semitendinosus muscle and to relate these changes to the decrease in the force-generating ability of the muscle. Trains of APs were recorded before and after stimulation (100-ms trains, 150 Hz, 1/s for 5 min). The resting membrane potential (RMP), AP overshoot (OS), and duration at 50% of peak magnitude (DUR) were -84.3 +/- 2.0 mV, 19.5 +/- 1.9 mV, and 1.3 +/- 0.1 ms, respectively, before stimulation. The stimulation protocol caused RMP to depolarize to -75.1 +/- 2.0 mV, OS to fall to 7.3 +/- 1.9 mV, and DUR to increase to 2.5 +/- 0.4 ms. RMP and OS recovered fully in 5 min after the cessation of stimulation, whereas DUR was still prolonged. Before the stimulation protocol, AP frequency matched the stimulation frequency at all stimulation rates < or = 150 Hz. At 200-Hz stimulation, AP frequency was 192 +/- 6 Hz. After 5 min of stimulation, AP frequency matched the stimulation frequency only at < or = 60 Hz. At 100-, 150-, and 200-Hz stimulation, AP frequencies were 89 +/- 8, 84 +/- 17, and 79 +/- 15 Hz, respectively. Because of a decreased fusion frequency at fatigue, the fall in the sarcolemma AP frequency did not contribute to the decreased force. The stimulation-induced alterations in the AP waveform were moderate and unlikely to have caused fatigue. However, the alterations in AP may have been more extreme in the depths of the transverse tubules.

Fatigability and blood flow in the rat gastrocnemius-plantaris-soleus after hindlimb suspension.

The purpose of this study was to test the hypothesis that hindlimb suspension increases the fatigability of the soleus during intense contractile activity and determine whether the increased fatigue is associated with a reduced muscle blood flow. Cage-control (C) and 15-day hindlimb-suspended (HS) rats were anesthetized, and either the gastrocnemius-plantaris-soleus (G-P-S) muscle group or the soleus was stimulated (100 Hz, 100-ms trains at 120/min) for 10 min in situ. In the G-P-S preparation, blood flow was measured with radiolabeled microspheres before and at 2 and 10 min of contractile activity. The G-P-S fatigued markedly at this stimulation frequency, and the differences between C and HS animals were not significant until the 9th min of contractile activity. In contrast, the stimulation resulted in faster rates and significantly larger amounts of fatigue in the soleus from HS than from C animals. The atrophied soleus showed significant differences by 1 min of stimulation (C = 70 +/- 1% vs. HS = 57 +/- 2% of peak train force) and remained different at 10 min (C = 64 +/- 4% vs. HS = 45 +/- 2% peak train force). Relative blood flow to the soleus was similar between groups before and during contractile activity (rest: C = 20 +/- 3 vs. HS = 12 +/- 3; 2 min: C = 128 +/- 6 vs. HS = 118 +/- 4; 10 min: C = 123 +/- 11 vs. HS = 105 +/- 11 ml.min-1.100 g-1). In conclusion, these results established that 15 days of HS increased the fatigability of the soleus, but the effect was not caused by a reduced muscle blood flow.

Effect of high-intensity exercise training on functional capacity of limb skeletal muscle.

The purpose of this study was to determine whether a program of regular sprint exercise training alters the functional properties or protects against the development of fatigue in fast- and slow-twitch rat skeletal muscle. The training program consisted of 6 sprints of 4.5-min duration at 40 m/min and 15% slope with 2.5-min rest intervals, performed 5 days/wk for 6 wk. The exercise program significantly increased (P less than 0.05) citrate synthase activity (mumol X g-1 X min-1) in the predominantly type I soleus (SOL) from 28 +/- 2 to 44 +/- 2; the type IIb superficial region of the vastus lateralis (SVL) from 10 +/- 1 to 16 +/- 1; and the type IIa deep region of the vastus lateralis (DVL) from 34 +/- 2 to 53 +/- 2. Phosphofructokinase activity (mumol X g-1 X min-1) also increased with training in the SOL (17 +/- 1 vs. 23 +/- 1) and the DVL (64 +/- 5 vs. 79 +/- 5). Sprint training reduced (P less than 0.05) the contraction time (CT) (111 +/- 7 vs. 92 +/- 3 ms) and the one-half relaxation time (118 +/- 3 vs. 104 +/- 2 ms) in the slow-twitch soleus. The exercise program also induced a decreased CT in the fast-twitch extensor digitorum longus (EDL), but significance was limited to the P less than 0.1 level. Muscle fatigue was produced by electrical stimulation at 45 trains/min and either 15 trains/min in SOL or 10 trains/min in the EDL and SVL for 1, 5, or 10 min.(ABSTRACT TRUNCATED AT 250 WORDS)